The present invention relates to a separator which is one of components in a fuel cell, and also relates to a fuel cell using thereof.
There have been several kinds of fuel cells which are sorted in view of kinds of electrolytes used therein. For example, a phosphate acid fuel cell (PAFC) has a carrier impregnated therein with phosphate, and is adapted to be operated at a temperature in a range from 150 to 220 deg. C. A molten carbonate fuel cell (MCFC) includes a molded electrolyte carrier made of a mixture of lithium carbonate and potassium carbonate, and is adapted to be operated at a temperature in a range from 600 to 700 deg. C. Further, a solid oxide fuel cell uses, as electrolyte, stabilized zirconium having oxygen ion conductivity, and is adapted to be operated at a temperature from 700 to 1,000 deg. C. Any of the above-mentioned fuel cells utilizes hydrogen, reformed gas, hydrocarbon or the like as a fuel, and air or the like as oxidizer gas.
Among several kinds of fuel cells, a proten exchange membrane fuel cell (PEMFC) or a direct methanol fuel cell (DMFC) mainly has such a feature that a membrane-like solid electrolyte made of polymer is jointed thereto at its opposite surfaces with carbon electrodes carrying catalyst such as platinum. This will be referred to a membrane electrode assembly (an electrolytic membrane/electrode integral structure) which is abbreviated to “MEA”. The solid polymer electrolyte fuel cell has such a configuration that the MEA is interposed between a pair of panels called as separators and formed therein with passages for fuel gas (containing hydrogen) and oxidizer gas (oxygen or air).
It is noted that the fluid such as gas as fuel and the fluid such as gas as an oxidizer will be inclusively referred to as reaction gas or reaction fluid. In general, a porous carbon sheet is interposed between the MEA and the separator. This constitutes a gas diffusion layer which can enhance such a function that the reaction gas is efficiently and uniformly fed to electrodes. The above-mentioned components are bundled into a set which is called as a unit cell, and a fuel cell stack is composed of a plurality of unit cells stacked one upon another. The separator has such a roll that the reaction gas is efficiently fed to electrodes, and therefore, when the reaction gas is fed to a fuel cell while an suitable load is applied, an electric power can be produced. In association, heat such as heat of reaction and Joule's heat is also generated. In order to remove the heat, the fuel cell incorporates, in general, a separator for feeding cooling water which passes through a part of the above-mentioned separator.
A separator also has a roll of transferring electric power between adjacent cells with less energy loss, and accordingly, it is, in general, made of carbon group conductive materials and is formed therein with passage channels for ventilating reaction gas and passing cooling medium. It has been considered that a metal sheet or the like is used as a material of the separator as a separator material, in addition to the carbon group material. Since a metal has a low material cost, and can be simply fabricated by stamping, and since a thin sheet metal can be used, it can offer such a merit that the separator can be compact and lightweight, and such a feature that the costs thereof are reduced.
However, in the case of a separator made of metal, should a thin sheet metal be formed therein with passage channels by pressing, it would be difficult to obtain fluid passages having desired depths and widths due to a limitation of workability caused by a process limit to a metal material. Thus, there would be caused such hindrances as non-uniformity of reaction gas streams, and less area of contact with an electrode. As a result, there would be caused such a problem that a desired power generating performance cannot be obtained. Even though a desired channel can be formed, the separator after fabrication would be warped or deformed, or could not have a required degree of finishing accuracy, resulting in leakage of reaction gas or increase in contact resistance.
As another disadvantage caused by the press-formed metal separator, apex tops of channels after fabrication have curvatures, and accordingly, an area of contact with the gas diffusion layer or the like becomes less. As a result, there would be caused such a problem that the resistance is increased.
In the case of making conventional metal separators in contact with one another, there would be caused such a problem that their contact area cannot be obtained sufficiently. That is, since the apices of channels for passage of reaction gas, which are defined by spaces between two separators mated with one another, are not flat, the separators are made into line or point contact with one another, and accordingly, the resistance of contact becomes higher, resulting in difficulty in obtaining a satisfactory performance of power generation. In order to eliminate the above-mentioned problem, JP-A-2003-173791 discloses such a configuration that parts of apices having curvatures are removed so as to be flattened. Further, JP-A-2003-123801 discloses such a configuration that a conductive sheet gasket is interposed between contact surfaces of separators in order to prevent occurrence of voltage drop caused by a resistance of contact at surfaces of cooling water between the separators.
As one of conventional inventions which can effectively solve the above-mentioned problems, there is a separator as disclosed in JP-A-2000-123850 or JP-A-2000-294257. This separator is composed of a metal thin sheet and a carbon paper which is cut so as to form passages in order to serve as a gas passage member. Thus, a single separator can be obtained without press-forming, and accordingly, it can reduce the costs. Further, since the passage part is formed by cutting the carbon paper, the degree of finishing accuracy is high, and further, it has a flat surface making contact with a gas diffusion layer, thereby it is possible to eliminate the above-mentioned problems.
The separator composed of the thin metal sheet and the carbon paper which is cut so as to form passages in order to serve as the gas passage member, as disclosed in JP-A-2000-123850 or JP-A-2003-123801 have several advantages. However, the carbon paper forming the passage part is split into several members, the larger the number of passages, the larger the number of subdivided passage members. As a result, there have been such problems that the number of components constituting a cell is increased, and that the number of manufacturing steps is increased since the components are fastened to one another by conductive materials. Further, in the inventions stated in the above-mentioned patent documents, no countermeasures are considered against corrosion on the metal side which is caused at contact surfaces of the metal separator and the passage part. Thus, there would be caused increase in contact resistance caused by corrosion on the metal side, contamination to electrodes and electrolytic membranes caused by corrosion products, and the like, resulting in deterioration of the fuel cell.